Factors that alter metabolism, including concomitant AEDs that may have hepatic enzyme induction effects will be tracked but not excluded, as long as the dose remains constant, as the goal is to reproduce the 'real life' situation as closely as possible within the practical limits of funding and study size. Subjects receiving valproate as concomitant medication will be excluded because the prolongation of half life would not allow LTG to reliably be completely cleared by 13 days. The criteria that establish an enriched population will also be tracked and used in a secondary analysis, but will not be used as an inclusion requirement. The enriched population is defined as people who experienced an otherwise unexplained increase in seizures or adverse effects, or a substantial change in AED level after a switch in AED products.